<<Disclaimer:  Verify this information before applying it to your situation.>>

While a true "all-beef" patty is fine -- do be aware of hamburgs with
fillers.  We find that many times restaurants buy hamburgs with fillers --
Ask before ordering -- "I'm on a VERY struct diet - can't eat gluten".  If
you say "wheat", they will look at ingredients and miss modified food starch,
oats, etc.  Tell them you want a list of the ingredients from the box.  If
they can't give it to you, you are taking a chance.

By the way, McDonald's and Burger King typically have ingredients lists --
their burgers are ok if right off the grill.  Just tell them "a plain patty
right off the grill, don't take it off the bun please, even a crumb will make
me very sick!"  It's ok to scare them a little if it will make them more
concientious.